1. Field of the Invention
The present invention relates to photoreceptor semiconductor circuits. More particularly, the present invention relates to adaptive photoreceptor semiconductor circuits for long-time-constant continuous adaptation having low offset and insensitivity to light, and to adaptive elements for employment therein.
2. The Prior Art
Semiconductor photoreceptor circuits are known in the art. The precursor for the present inventions is a logarithmic photoreceptor described in C. A. Mead, A Sensitive Electronic Photoreceptor, 1985 Chapel Hill Conference on VLSI, H. Fuchs Ed., Rockville: Computer Science Press, pp. 463-471, 1985, used in the early silicon retinas described in M. A. Mahowald and C. A. Mead, "Silicon Retina," in Analog VLSI and Neural Systems, by C. Mead, Reading: Addison-Wesley, pp. 257-278, 1989, and C. A. Mead and M. A. Mahowald, "A Silicon Model of Early Visual Processing," Neural Networks, vol. 1, pp. 91-97, 1998, and in the SeeHear chip disclosed in L. Nielson, M. Mahowald, C. A. Mead, "SeeHear," in Analog VLSI and Neural Systems, by C. Mead, Reading: Addison-Wesley, chapter 13, pp. 207-227, 1989. (adapted there from 1987 International Association for Pattern Recognition, 5th Scandinavian Conference on Image Analysis.) These receptors have at least three deficiencies including the poor matching between different receptors, the poor match between the low sensitivity of the receptor and the high gain required to sense the small contrasts present in real images, and a slow time-response that limits the dynamic range.
An adaptive receptor that had a large transistor count, lacked any speedup advantage of the active feedback, and used an inferior adaptive element is described in T. Delbruck and C. A. Mead, "An electronic photoreceptor sensitive to small changes in intensity," in Advances in Neural Information Processing Systems I, D. S. Touretzky, Ed., San Mateo: Morgan Kaufman, pp. 720-727, 1988.
In addition, an adaptive silicon retina for sensing time-derivatives of the image contrast, using feedback is described in T. Delbruck and C. A. Mead, (1991), Silicon adaptive photoreceptor array that computes temporal intensity derivatives, in T. S. Jay Jayedev (ed,) Proc. SPIE, Infrared Sensors: Detectors, Electronics, and Signal Processing., vol. 1541, pp 92-99. A silicon retina photoreceptor incorporating feedback is described in M. A. Mahowald, "Silicon Retina with Adaptive Photoreceptors," Proc. SPIE./SPSE Symposium on Electronic Science and Technology: from Neurons to Chips, vol. 1473, pp. 52-58, April 1991. At about the same time, Mann, J. Mann "Implementing Early Visual Processing in Analog VLSI: Light Adaptation," Proc. SPIE/SPSE Symposium on Electronic Science and Technology: from Neurons to Chips, vol. 1473, pp. 128-136, April 1991, developed several adaptive photoreceptor circuits that are flexible but use a large number of components and were never fully reduced to practice. All of these receptors also lack the advantage of active speedup and use inferior adaptive elements. None of the previous receptors were satisfactorily characterized, in the sense that the simple engineering metrics like usable dynamic range and sensitivity were not obtained.
Prior art photosensitive devices are known and used in commercial imager arrays but are not particularly pertinent to the present invention. Such detectors are designed to be sampled in time, rather than operate in continuous time, and many of these devices cannot be built in a standard CMOS process.